1. Field
The present invention relates generally to communications systems, and more specifically, to systems and techniques for acquisition of a gated pilot signal.
2. Background
Modern communications systems are designed to allow multiple users to share a common communications medium. One such communications system is a code division multiple access (CDMA) system. The CDMA communications system is a modulation and multiple access scheme based on spread-spectrum communications. In a CDMA communications system, a large number of signals share the same frequency spectrum and, as a result, provide an increase in user capacity. This is achieved by transmitting each signal with a different pseudo-noise (PN) code that modulates a carrier, and thereby, spreads the spectrum of the signal waveform. The transmitted signals are separated in the receiver by a correlator that uses a corresponding PN code to despread the desired signal's spectrum. The undesired signals, whose PN codes do not match, are not despread in bandwidth and contribute only to noise.
In a CDMA communications system, a subscriber station may access a network, or communicate with other subscriber stations, through one or more base stations. Each base station is configured to serve all subscriber stations in a specific geographic region generally referred to as a cell. In some high traffic applications, the cell may be divided into sectors with a base station serving each sector. Each base station transmits a continuous pilot signal which is used by the subscriber stations for synchronizing with a base station and to provide coherent demodulation of the transmitted signal once the subscriber station is synchronized to the base station. The subscriber station generally establishes a communications channel with the base station having the strongest pilot signal.
Since a continuous pilot signal requires bandwidth that could otherwise be used to transmit information, some recently developed CDMA communications systems have employed gated pilot signals. A gated pilot signal is characterized by a short period of transmission of pilot signal followed by a long period of no transmission. By gating the pilot signal, additional bandwidth can be realized which increases the capacity of the base station. However, synchronizing the subscriber station to the gated pilot signal is relatively more difficult than synchronizing the subscriber station to a continuous pilot signal because in a gated pilot system, the subscriber station may spend considerable resources searching for the pilot signal during periods when none is present.
Gated pilot signals with overlapping pilot bursts in PN space can cause relatively strong partial correlations in adjacent PN offsets. These partial correlation peaks can be falsely assumed as the real PN offsets. A subscriber station may waste significant time in achieving frequency lock with and unsuccessfuly trying to demodulate the control channel from these non-existent PN offsets. Frequency locking and control channel demodulation are the most time consuming steps in the acquisition process and typically take four to eight times more time and resources than the pilot search operation. Hence, these kinds of false alarms can potentially increase the acquisition time by a significant factor. What is needed is a method to significantly reduce the probability of false alarms caused due to partial correlations between adjacent pilot bursts.